1. Field of the Invention
This invention relates to an apparatus for measuring the refractive index of liquid, and, in particular, to an apparatus for measuring the refractive index of such liquid as ink, which contains a dye as dispersed therein, suitable for use in an ink-jet printer, for example.
2. Description of the Prior Art
FIG. 12 shows a typical prior art apparatus for measuring the refractive index of liquid using a U-shaped optical fiber. As shown, the illustrated refractive index measuring apparatus includes a U-shaped optical fiber 1 having a light input end and a light output end, a light emitting element 2 located opposite to the light input end of the optical fiber 1, and a light receiving element 3 located opposite to the light outout end of the optical fiber 1. As is well known, in this apparatus, the curved portion of the U-shaped optical fiber 1 is immersed in a liquid whose refractive index is to be measured. In this case, the light emitted from the light emitting element 2 is introduced into the U-shaped optical fiber 1 through the light input end thereof, propagates through the optical fiber 1 and finally reaches the light receiving element 3. The amount of light which leaks out of the optical fiber 1 through its side surface varies depending on the refractive index of the liquid in contact with the optical fiber 1, so that the refractive index of the liquid in contact with the optical fiber 1 can be determined by measuring the amount of light emitted from the light output end of the optical fiber 1 by the light receiving element 3.
FIG. 13 shows another prior art apparatus for measuring the refractive index of liquid which is similar in structure to the apparatus shown in FIG. 12; however, in the apparatus shown in FIG. 13, the optical fiber 1 has a loop so as to improve the linearity in the characteristic. The structure shown in FIG. 13 is disclosed in the U.S. Pat. No. 3,282,149. FIG. 14 shows a graph which has its abscissa taken for the refractive index of liquid and its ordinate taken for the relative light output. In the graph of FIG. 14, the curve A indicates the output characteristic of the apparatus using the U-shaped optical fiber 1 shown in FIG. 12 and the curve B indicates the output characteristic of the apparatus using the loop shaped optical fiber 1 shown in FIG. 13. It is seen that the range of linearity in the output characteristic can be enlarged by using the loop-shaped optical fiber instead of the U-shaped optical fiber.
FIG. 15 shows a further prior art apparatus for measuring the refractive index of liquid which is disclosed in the Japanese Patent Laid-open Publication No. 56-73335. The apparatus shown in FIG. 15 is so structured to prevent the characteristic from deteriorating by the deposition of fine particles in the liquid to be measured onto the optical fiber 1. For this purpose, in the illustrated apparatus, the optical fiber 1 is enclosed in an enclosure 4, part of which is formed by a filter 5, thereby allowing the liquid to be measured to move into and out of the enclosure 4 freely, but to prevent fine particles and the like from entering into the interior of the enclosure 4. However, the structure shown in FIG. 15 tends to be bulky and is not effective for finer particles which pass through the filter 5. Besides, the characteristics of the light emitting and receiving elements vary depending on the disturbances in the abmient temperature, which constitute one of the factors for deteriorating the detection accuracy. It is to be noted that none of the prior art pays attention to the relation between the kind of liquid to be measured and the wavelength of a light source.